1. Field of the Invention
The present invention relates to an image forming apparatus capable of image formation under optimum image forming conditions in response to the detected image density.
2. Description of the Prior Art
There is already known an apparatus in which an original is illuminated with the light of a determined intensity and is scanned with a photosensor to detect, by the output signals thereof, the light intensities from the background area and image area of said original, whereby the image forming condition such as the exposure or developing bias is appropriately determined according to thus detected information.
In such apparatus, when the maximum output voltage of the photosensor is selected equal to the maximum level of the predetermined voltage range, the minimum output voltage of the photosensor for a usual original is several times smaller than said maximum level. Consequently the maximum value may become unmeasurable if the signal level is so selected to allow sufficiently precise measurement of the minimum value. On the other hand, the measurement of the minimum value may become insufficiently precise if the signal level is so selected to allow precise measurement of the maximum value. Particularly when the original consists of a negative image such as a microfilm, the precision of the automatic exposure adjusting function is often deteriorated since the exposure is generally determined by the maximum output voltage of the photosensor with reference to the minimum voltage thereof.
The appropriate exposure E for a film original is determined in the following manner in relation to the luminance B obtained by logarithmic conversion of the output S of the photosensor receiving the light transmitted by the original. An appropriate exposure En for a negative original is determined by: EQU En=.alpha..sub.1 B.sub.min +.beta..sub.1 (B.sub.max -B.sub.min)+.gamma..sub.1 ( 1)
wherein B.sub.min and B.sub.max are respectively minimum and maximum luminances obtained by logarithmic conversion of the outputs S.sub.min and S.sub.max receiving the lights transmitted by the background and image areas of the original, and .alpha..sub.1, .beta..sub.1 and .gamma..sub.1 are constants in which .alpha..sub.1 &lt;0 and .beta..sub.1 &lt;0. Similarly an appropriate exposure Ep for a positive original is determined by: EQU Ep=.alpha..sub.2 B.sub.max +.beta..sub.2 (B.sub.max -B.sub.min)+.gamma..sub.2 ( 2)
wherein .alpha..sub.2, .beta..sub.2 and .gamma..sub.2 are constants in which .alpha..sub.2 &gt;0 and .beta..sub.2 &gt;0.
However, linear calculation formulas such as (1) and (2) explained above may reduce the automatically adjustable density range in practice, since the appropriate exposure determined by these formulas may become different from the actually desirable exposure at a high or low image density depending on the characteristics of the photosensitive member, charger or developing bias. Particularly the contrast-dependent second correction term in the foregoing equations (1) and (2) may not be appropriate under certain process conditions and should be non-linearly modified according to whether the image contrast is high or low.
It is therefore considered to adopt non-linear equations for determining the appropriate exposure E by selecting suitable functions f(x) and g(y) for the first and second terms of the foregoing formulas (1) and (2). More specifically the appropriate exposure En Ep are determined by: EQU En=f(B.sub.min)+g(B.sub.max -B.sub.min)+K.sub.1 ( 3)
for a negative original, wherein K.sub.1 is a constant, and EQU Ep=f(B.sub.max)+g(B.sub.max -B.sub.min)+K.sub.2 ( 4)
for a positive original, wherein K.sub.2 is a constant. In this manner it is rendered possible to achieve automatic exposure control within the practically realizable range of density and contract in the process.
The above-mentioned formulas (3) and (4) can perform correct exposure control if the light is measured over the entire image area of the original, but the image line is not necessarily positioned in the measureing area if it is locally limited. The probability of presence of image lines within the light measuring area is quite low in certain originals such as patent drawings, so that the calculated exposure fluctuates depending on whether the image lines exist in the light measuring area since the second term of the formula (3) or (4) is a function of the contrast or difference in luminance between the background and image areas of the original. Thus the exact copy density control is not possible as the exposure varies depending on whether the image lines are present in the light measuring area at the detection of the image density. Such situation occurs also at the control of the copy density with other adjusting means.